The present invention is directed to a ceramic discharge vessel for a high-intensity discharge lamp, and more particularly to a single-ended discharge vessel with electrodes that diverge from each other so that a discharge arc is confined to tips of the electrodes.
FIG. 1 is a cross-sectional illustration of a single-ended discharge vessel of the prior art. The discharge vessel 10 includes a ceramic body 12, two capillaries 14 extending from a same side of body 12 (e.g., extending from a common hemisphere), and two electrodes 16 that are each in a different one of the two capillaries 14. The longitudinal axes of the capillaries 14 and the electrodes 16 are all in a common plane of the drawing sheet. European Patent Application 1 111 654 describes discharge vessels of this type. Capillaries 14 and electrodes 16 extend from the same side of body 12 to reduce the overall size of the discharge vessel compared to double-ended discharge vessels that have two capillaries that extend collinearly from opposite sides of the body.
One of the problems with this side-by-side arrangement of electrodes is that an arc discharge between the electrodes can walk down the electrodes toward the wall of the discharge vessel and damage the ceramic. To discourage this, the electrodes are angled apart so that they diverge from each other thereby making the tips of the electrodes the closest two points between the electrodes inside the discharge vessel. The electrodes 16 in FIG. 1 diverge from each other within the common plane to attempt to confine the arc discharge between electrodes 16 to electrode tips 18.
The amount of divergence of electrodes 16 from each other can be confined to a relatively small angular range (up to about 12°) in discharge vessels in which the distal ends of capillaries 14 do not project beyond an edge of body 12, as illustrated by the dashed lines E in FIG. 1. It would be desirable to increase this angular range while keeping the distal ends of capillaries 14 confined within the edge of body 12.
Further, it would be desirable to offer an attractive alternative arrangement of the capillaries to create an additional option for reducing the overall size of the discharge vessel.